Due to its penetrating but relatively non-damaging wavelengths, X-ray radiation is used in a variety of imaging applications. While X-ray imaging systems may utilize X-ray tubes collimated to emit a cone beam of X-rays toward a relatively large detector, imaging systems have been developed wherein the X-ray source can emit relatively thin beams of radiation from a plurality of discrete focal spots on its face, allowing for techniques that can extract more image information, reduced scatter noise on the detector, and lower patient radiation dose per image. One type of multi-focal spot source which has been used is a scanning beam source. An example of a scanning beam source is described in U.S. Pat. No. 5,682,412 issued to Skillicorn et al. entitled “X-ray Source.”
In X-ray tubes, X-rays may be produced by the incidence of high-energy, e.g. accelerated, charged particles on a targeted sheet of metal or other material; fast-moving particles can collide with particles within the target atoms and, in disturbing the ground state electron distribution of the atoms or interacting with the nuclear electric field, can cause X-ray fluorescence or bremsstrahlung X-ray radiation, respectively. In a scanning beam source, X-rays may be generated by these mechanisms. However, charged particles may strike a plurality of discrete locations on the target screen sequentially, rather than the entire screen at once, so that X-rays can be emitted from discrete focal spots.
A particle gun can be used in the source to generate, accelerate, and focus particles toward a target screen. Focusing charged particles into a beam can significantly increase the concentration, or density, of charged particles striking the target; in a point-source X-ray tube particles can strike the entire source face whereas in a scanning beam source particles may be concentrated in a small, localized area. High particle concentration may lead to target burnout, e.g. destruction by deposition of too much energy in too small of an area.
Furthermore, in point-source tubes, a uniform particle density can be achieved by focusing the beam at a point beyond the actual target screen. Even if a relatively narrow beam were required, e.g. a beam as narrow as a discrete focal spot, the same mechanism could be used to achieve a uniform particle density in the beam, though the point at which the beam is focused may be relatively nearer to the target screen. However, in scanning beam sources a narrow beam may need to be rapidly refocused on up to 9,000 discrete focal spots or more. As particle concentration may increase proportionally with distance from the source in a focused beam—the number of particles in a cross-section being constant, and the width of the beam decreasing to the focus—it can be difficult to maintain a particle concentration below the burnout threshold in the plane of the target screen while rapidly moving the beam between a plurality of focal spots located at unique distances from the source.
What is needed is a particle beam with a well-defined disk of uniformly distributed particles that can be focused on the target screen.